For vehicles employing a step-ratio or discrete ratio transmission, the transmission input shaft is constrained to be proportional to the vehicle speed with a finite set of gear ratios, except during the brief interval while the transmission is shifting from one ratio to another. As the vehicle is operated, a controller selects the appropriate gear ratio and a command is sent to the transmission to shift to a different one of the finite gear ratios. This shifting between finite gear ratios generally results in engine speed changes and provides different torque multiplication at the vehicle wheels but may also result in “shift shock” as the transmission changes gear ratios.
Hybrid electric vehicles commonly utilize electronic continuously variable transmissions that do not impose a finite set of gear ratios between the transmission input shaft and transmission output shaft. The hybrid electric vehicle is operated by a controller that chooses an exact gear ratio along a variable continuum without gear changes and the associated “shift shock.” Additionally, various powertrain configurations do not mechanically impose a strict relationship between the engine speed and the vehicle speed so that engine speed may be controlled independently of the vehicle speed and transmission gear ratio.
For vehicles equipped with an automatic transmission, selection of the gear ratio or engine speed is ordinarily determined by a controller based on current vehicle and ambient operating conditions. Some drivers may prefer to control gear ratio selection and override the controller. Such vehicles may be equipped with driver interface features which permit the driver to signal a desire for a higher or a lower gear ratio relative to the gear ratio automatically selected by the vehicle controller to operate the transmission similar to a manual transmission, but still subject to various limits imposed by the controller to prevent engine overspeed or stall, for example. In a step-ratio automatic transmission, the controller shifts the transmission to one of a plurality of discrete gear ratios, which adjust engine speed accordingly and provide associated torque multiplication at the vehicle wheels. In vehicles equipped with a continuously variable transmission or similar gearbox, the response to a driver initiated shift command is more complicated because the transmission does not inherently provide discrete gear ratios with associated different torque multiplications. Therefore, the discrete gear ratios may be simulated and are sometimes referred to as virtual gears or gear ratios.
When transitioning from a continuously-variable mode to a manual or override mode, the initial discrete virtual gear ratio must be determined or selected. Because the virtual gear ratios are not constrained by traditional engine to vehicle speed ratios, the controller often has more than one suitable initial virtual gear selection based on current vehicle and ambient operating conditions and operator input, such as accelerator pedal position, gear selector position, paddle shifters, etc. However, driver expectations of vehicle behavior are generally different for an upshift compared to a downshift.